It’s about evolution; if dads aren’t around, mothers-to-be make subconscious calculations to balance their needs against those of their children.

In a report published in the American Journal of Human Biology, researchers say that in U.S. counties where women outnumber men, more babies are born underweight.

Researchers at the University of Michigan School of Public Health, led by assistant professor Daniel Kruger, compared birth records at the county level in 2000 from the U.S. Centers for Disease Control and Prevention against the 2000 U.S. Decennial Census data. They documented the proportion of babies that were born premature, at less than 37 weeks gestation, and of low birth weight (less 5.5 pounds). They also analyzed the ratio of men to women between ages 18 and 64, the proportion of single-mother households compared to families with children, and weighed socioeconomic status, among other factors.

Across 450 counties nationwide, the researchers found a direct association between higher rates of low birth weight and areas where women outnumbered men. Counties with a higher proportion of African-Americans appeared to have a greater degree of male scarcity.

While there are certainly medical explanations for low birth weight — risk factors such as smoking during pregnancy and inadequate prenatal care can contribute to smaller babies — the results may also support a central principle in evolutionary biology known as “life history tradeoffs,” according to Kruger. This framework puts low birth weight in the context of resources that the mother has at her disposal, and the likelihood of the baby’s survival. Mothers, according to this theory, make intuitive tradeoffs based on their living conditions. If men are scarce, and not likely to remain as supportive partners in raising children, then mothers face a balancing act, forced to divide their limited energy and resources among raising their offspring and preserving their own bodies to reproduce again. The circumstance reflects how each species has a “finite amount of effort” and therefore has to make “tradeoffs between different aspects of life” in terms of growth, development, reproduction, and parenting, says Kruger. But the decisions may have long term consequences; babies born at low birth weight may be at an increased risk of certain health conditions later in life, including heart disease, diabetes and high blood pressure.

At the same time, if men are outnumbered by women, then they may choose to devote their efforts to having multiple partners and less on parenting. “You would think if men are scarce, it’s easier for them to get a partner and get married and have a family,” says Kruger, “but what’s happening at the population level is that men are less likely to settle down and get married, and more likely to use that leverage to play the field and get more partners. They don’t have to commit as readily as men in other populations because the women don’t have as much bargaining power.”

Kruger believes women have a subconscious “system” of regulating birth outcomes that has evolved over thousands of years — a type of protection that dates back to periods of history when childbirth was riskier and infant mortality was higher. In fact, anthropologists have shown that, in hunter-gatherer populations, babies who grew up without fathers were more likely to die before they reach maturity.

So when fewer contributions from fathers are coming in — dads supporting families with food, social support, and training skills — women may adjust accordingly. “When conditions seem unfavorable, women might reduce their investment in their current offspring, so they can save resources for [future] children who will have a better chance to survive and reproduce,” says Kruger. “Women are [not] consciously thinking, ‘the father of my child isn’t as likely to be around when the kid is born and growing up, so therefore, I’m going to have a premature baby.'” But evolution may have provided them with an innate ability to sense and accommodate their situation and make tradeoffs. In other words, “do you put all of your eggs in one basket or save some energy for later, when conditions might be better?” says Kruger.

The study highlights some of the evolutionary forces that might still be at work in helping us to survive at the species level. Individual decisions by mothers can, and certainly do, counter the trend, but the data support the fact that regardless of why, communities in which mothers are raising children on their own may need additional support in the form of economic or child care assistance. Absent that, even emotional support can be important to reversing the evolutionary forces that favor smaller, low-birth weight babies. Mothers, says Kruger, need to feel a “sense that someone cares for you and is there for you,” and that a man is “willing to help out, teach [children] skills, and take the burden off of the mother.”